diff --git a/Cargo.toml b/Cargo.toml
index e5a0010..7e98ab5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -39,6 +39,7 @@ tracing = "0.1"
 bytes = "1"
 dirs = "5"
 blake3 = "1"
+hex = "0.4"
 
 # Testing
 tokio-test = "0.4"
diff --git a/lattice-core/Cargo.toml b/lattice-core/Cargo.toml
index 0c0cbc0..7696c22 100644
--- a/lattice-core/Cargo.toml
+++ b/lattice-core/Cargo.toml
@@ -13,6 +13,7 @@ thiserror = { workspace = true }
 bytes = { workspace = true }
 dirs = { workspace = true }
 blake3 = { workspace = true }
+hex = { workspace = true }
 
 [build-dependencies]
 prost-build = { workspace = true }
diff --git a/lattice-core/src/sigchain.rs b/lattice-core/src/sigchain.rs
index 65f96fe..42e8c87 100644
--- a/lattice-core/src/sigchain.rs
+++ b/lattice-core/src/sigchain.rs
@@ -1,20 +1,430 @@
 //! Cryptographic SigChain (append-only signed log)
+//!
+//! A SigChain manages a single author's append-only log. It validates entries
+//! before appending (correct seq, prev_hash, valid signature) and persists to disk.
+
+use crate::log::{append_entry, read_entries, LogError};
+use crate::node::Node;
+use crate::proto::{Entry, SignedEntry};
+use crate::signed_entry::{hash_signed_entry, verify_signed_entry};
+use prost::Message;
+use std::path::{Path, PathBuf};
+use thiserror::Error;
+
+/// Errors that can occur during sigchain operations
+#[derive(Error, Debug)]
+pub enum SigChainError {
+    #[error("Log error: {0}")]
+    Log(#[from] LogError),
+    
+    #[error("Invalid signature")]
+    InvalidSignature,
+    
+    #[error("Wrong author: expected {expected}, got {got}")]
+    WrongAuthor { expected: String, got: String },
+    
+    #[error("Invalid sequence: expected {expected}, got {got}")]
+    InvalidSequence { expected: u64, got: u64 },
+    
+    #[error("Invalid prev_hash: expected {expected}, got {got}")]
+    InvalidPrevHash { expected: String, got: String },
+    
+    #[error("Decode error: {0}")]
+    Decode(#[from] prost::DecodeError),
+}
 
 /// An append-only log where each entry is cryptographically signed
 /// and hash-linked to the previous entry.
 pub struct SigChain {
-    // TODO: entries, hash chain
+    /// Path to the log file
+    log_path: PathBuf,
+    
+    /// Author's public key (32 bytes)
+    author_id: [u8; 32],
+    
+    /// Next expected sequence number
+    next_seq: u64,
+    
+    /// Hash of the last entry (zeroes if empty)
+    last_hash: [u8; 32],
 }
 
 impl SigChain {
-    /// Create a new empty sigchain.
-    pub fn new() -> Self {
-        Self {}
+    /// Create a new empty sigchain for an author
+    pub fn new(log_path: impl AsRef<Path>, author_id: [u8; 32]) -> Self {
+        Self {
+            log_path: log_path.as_ref().to_path_buf(),
+            author_id,
+            next_seq: 1,
+            last_hash: [0u8; 32],
+        }
+    }
+    
+    /// Load a sigchain from an existing log file
+    pub fn from_log(log_path: impl AsRef<Path>, author_id: [u8; 32]) -> Result<Self, SigChainError> {
+        let log_path = log_path.as_ref().to_path_buf();
+        let entries = read_entries(&log_path)?;
+        
+        let mut chain = Self::new(&log_path, author_id);
+        
+        for signed_entry in entries {
+            // Verify signature
+            verify_signed_entry(&signed_entry)
+                .map_err(|_| SigChainError::InvalidSignature)?;
+            
+            // Validate author (author_id is in SignedEntry)
+            let entry_author: [u8; 32] = signed_entry.author_id.clone().try_into()
+                .unwrap_or([0u8; 32]);
+            if entry_author != author_id {
+                return Err(SigChainError::WrongAuthor {
+                    expected: hex::encode(author_id),
+                    got: hex::encode(&entry_author),
+                });
+            }
+            
+            // Decode Entry
+            let entry = Entry::decode(&signed_entry.entry_bytes[..])?;
+            
+            // Validate sequence
+            if entry.seq != chain.next_seq {
+                return Err(SigChainError::InvalidSequence {
+                    expected: chain.next_seq,
+                    got: entry.seq,
+                });
+            }
+            
+            // Validate prev_hash
+            let expected_prev: [u8; 32] = chain.last_hash;
+            let got_prev: [u8; 32] = entry.prev_hash.try_into()
+                .unwrap_or([0u8; 32]);
+            if got_prev != expected_prev {
+                return Err(SigChainError::InvalidPrevHash {
+                    expected: hex::encode(expected_prev),
+                    got: hex::encode(got_prev),
+                });
+            }
+            
+            // Update state
+            chain.last_hash = hash_signed_entry(&signed_entry);
+            chain.next_seq += 1;
+        }
+        
+        Ok(chain)
+    }
+    
+    /// Get the author's public key
+    pub fn author_id(&self) -> &[u8; 32] {
+        &self.author_id
+    }
+    
+    /// Get the next expected sequence number
+    pub fn next_seq(&self) -> u64 {
+        self.next_seq
+    }
+    
+    /// Get the hash of the last entry
+    pub fn last_hash(&self) -> &[u8; 32] {
+        &self.last_hash
+    }
+    
+    /// Get the current length of the chain
+    pub fn len(&self) -> u64 {
+        self.next_seq - 1
+    }
+    
+    /// Check if the chain is empty
+    pub fn is_empty(&self) -> bool {
+        self.next_seq == 1
+    }
+    
+    /// Validate a signed entry without appending
+    pub fn validate(&self, signed_entry: &SignedEntry) -> Result<(), SigChainError> {
+        // Verify signature
+        verify_signed_entry(signed_entry)
+            .map_err(|_| SigChainError::InvalidSignature)?;
+        
+        // Validate author (author_id is in SignedEntry)
+        let author: [u8; 32] = signed_entry.author_id.clone().try_into()
+            .unwrap_or([0u8; 32]);
+        if author != self.author_id {
+            return Err(SigChainError::WrongAuthor {
+                expected: hex::encode(self.author_id),
+                got: hex::encode(author),
+            });
+        }
+        
+        // Decode entry
+        let entry = Entry::decode(&signed_entry.entry_bytes[..])?;
+        
+        // Validate sequence
+        if entry.seq != self.next_seq {
+            return Err(SigChainError::InvalidSequence {
+                expected: self.next_seq,
+                got: entry.seq,
+            });
+        }
+        
+        // Validate prev_hash
+        let prev: [u8; 32] = entry.prev_hash.try_into()
+            .unwrap_or([0u8; 32]);
+        if prev != self.last_hash {
+            return Err(SigChainError::InvalidPrevHash {
+                expected: hex::encode(self.last_hash),
+                got: hex::encode(prev),
+            });
+        }
+        
+        Ok(())
+    }
+    
+    /// Append a signed entry to the chain (validates first)
+    pub fn append(&mut self, signed_entry: &SignedEntry) -> Result<(), SigChainError> {
+        // Validate
+        self.validate(signed_entry)?;
+        
+        // Write to log
+        append_entry(&self.log_path, signed_entry)?;
+        
+        // Update state
+        self.last_hash = hash_signed_entry(signed_entry);
+        self.next_seq += 1;
+        
+        Ok(())
+    }
+    
+    /// Create and append a new entry using the node's key
+    pub fn create_entry(&mut self, node: &Node, ops: Vec<crate::proto::Operation>) -> Result<SignedEntry, SigChainError> {
+        use crate::clock::SystemClock;
+        use crate::hlc::HLC;
+        use crate::signed_entry::EntryBuilder;
+        
+        let hlc = HLC::now_with_clock(&SystemClock);
+        
+        let mut builder = EntryBuilder::new(self.next_seq, hlc)
+            .prev_hash(self.last_hash.to_vec());
+        
+        // Add operations
+        for op in ops {
+            builder = builder.operation(op);
+        }
+        
+        let signed = builder.sign(node);
+        
+        self.append(&signed)?;
+        
+        Ok(signed)
     }
 }
 
-impl Default for SigChain {
-    fn default() -> Self {
-        Self::new()
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::clock::MockClock;
+    use crate::hlc::HLC;
+    use crate::node::Node;
+    use crate::proto::{operation, Operation, PutOp};
+    use crate::signed_entry::EntryBuilder;
+    use std::env::temp_dir;
+
+    fn temp_log_path(name: &str) -> PathBuf {
+        temp_dir().join(format!("lattice_sigchain_test_{}.log", name))
+    }
+
+    #[test]
+    fn test_new_sigchain() {
+        let path = temp_log_path("new");
+        let author = [1u8; 32];
+        
+        let chain = SigChain::new(&path, author);
+        
+        assert_eq!(chain.author_id(), &author);
+        assert_eq!(chain.next_seq(), 1);
+        assert_eq!(chain.last_hash(), &[0u8; 32]);
+        assert!(chain.is_empty());
+        assert_eq!(chain.len(), 0);
+    }
+
+    #[test]
+    fn test_append_entry() {
+        let path = temp_log_path("append");
+        std::fs::remove_file(&path).ok();
+        
+        let node = Node::generate();
+        let author = node.public_key_bytes();
+        let mut chain = SigChain::new(&path, author);
+        
+        let clock = MockClock::new(1000);
+        let entry = EntryBuilder::new(1, HLC::now_with_clock(&clock))
+            .prev_hash([0u8; 32].to_vec())
+            .put("/key", b"value".to_vec())
+            .sign(&node);
+        
+        chain.append(&entry).unwrap();
+        
+        assert_eq!(chain.next_seq(), 2);
+        assert_eq!(chain.len(), 1);
+        assert!(!chain.is_empty());
+        
+        std::fs::remove_file(&path).ok();
+    }
+
+    #[test]
+    fn test_append_multiple() {
+        let path = temp_log_path("multiple");
+        std::fs::remove_file(&path).ok();
+        
+        let node = Node::generate();
+        let author = node.public_key_bytes();
+        let mut chain = SigChain::new(&path, author);
+        let clock = MockClock::new(1000);
+        
+        for i in 1..=3 {
+            let entry = EntryBuilder::new(i, HLC::now_with_clock(&clock))
+                .prev_hash(chain.last_hash.to_vec())
+                .put(format!("/key/{}", i), format!("value{}", i).into_bytes())
+                .sign(&node);
+            chain.append(&entry).unwrap();
+        }
+        
+        assert_eq!(chain.len(), 3);
+        assert_eq!(chain.next_seq(), 4);
+        
+        std::fs::remove_file(&path).ok();
+    }
+
+    #[test]
+    fn test_from_log() {
+        let path = temp_log_path("from_log");
+        std::fs::remove_file(&path).ok();
+        
+        let node = Node::generate();
+        let author = node.public_key_bytes();
+        let clock = MockClock::new(1000);
+        
+        // Write some entries
+        {
+            let mut chain = SigChain::new(&path, author);
+            for i in 1..=3 {
+                let entry = EntryBuilder::new(i, HLC::now_with_clock(&clock))
+                    .prev_hash(chain.last_hash.to_vec())
+                    .put("/key", b"val".to_vec())
+                    .sign(&node);
+                chain.append(&entry).unwrap();
+            }
+        }
+        
+        // Reload from log
+        let chain = SigChain::from_log(&path, author).unwrap();
+        
+        assert_eq!(chain.len(), 3);
+        assert_eq!(chain.next_seq(), 4);
+        
+        std::fs::remove_file(&path).ok();
+    }
+
+    #[test]
+    fn test_reject_wrong_sequence() {
+        let path = temp_log_path("wrong_seq");
+        std::fs::remove_file(&path).ok();
+        
+        let node = Node::generate();
+        let author = node.public_key_bytes();
+        let mut chain = SigChain::new(&path, author);
+        let clock = MockClock::new(1000);
+        
+        // Try to append with wrong seq (2 instead of 1)
+        let entry = EntryBuilder::new(2, HLC::now_with_clock(&clock))
+            .prev_hash([0u8; 32].to_vec())
+            .put("/key", b"val".to_vec())
+            .sign(&node);
+        
+        let result = chain.append(&entry);
+        
+        assert!(matches!(result, Err(SigChainError::InvalidSequence { .. })));
+        
+        std::fs::remove_file(&path).ok();
+    }
+
+    #[test]
+    fn test_reject_wrong_prev_hash() {
+        let path = temp_log_path("wrong_prev");
+        std::fs::remove_file(&path).ok();
+        
+        let node = Node::generate();
+        let author = node.public_key_bytes();
+        let mut chain = SigChain::new(&path, author);
+        let clock = MockClock::new(1000);
+        
+        // First entry
+        let entry1 = EntryBuilder::new(1, HLC::now_with_clock(&clock))
+            .prev_hash([0u8; 32].to_vec())
+            .put("/key", b"v1".to_vec())
+            .sign(&node);
+        chain.append(&entry1).unwrap();
+        
+        // Second entry with wrong prev_hash
+        let entry2 = EntryBuilder::new(2, HLC::now_with_clock(&clock))
+            .prev_hash([99u8; 32].to_vec()) // Wrong!
+            .put("/key", b"v2".to_vec())
+            .sign(&node);
+        
+        let result = chain.append(&entry2);
+        
+        assert!(matches!(result, Err(SigChainError::InvalidPrevHash { .. })));
+        
+        std::fs::remove_file(&path).ok();
+    }
+
+    #[test]
+    fn test_reject_wrong_author() {
+        let path = temp_log_path("wrong_author");
+        std::fs::remove_file(&path).ok();
+        
+        let node = Node::generate();
+        let other_author = [99u8; 32]; // Different author
+        let mut chain = SigChain::new(&path, other_author);
+        let clock = MockClock::new(1000);
+        
+        // Entry signed by node but chain expects other_author
+        let entry = EntryBuilder::new(1, HLC::now_with_clock(&clock))
+            .prev_hash([0u8; 32].to_vec())
+            .put("/key", b"val".to_vec())
+            .sign(&node);
+        
+        let result = chain.append(&entry);
+        
+        assert!(matches!(result, Err(SigChainError::WrongAuthor { .. })));
+        
+        std::fs::remove_file(&path).ok();
+    }
+
+    #[test]
+    fn test_create_entry() {
+        let path = temp_log_path("create");
+        std::fs::remove_file(&path).ok();
+        
+        let node = Node::generate();
+        let author = node.public_key_bytes();
+        let mut chain = SigChain::new(&path, author);
+        
+        let ops = vec![
+            Operation {
+                op_type: Some(operation::OpType::Put(PutOp {
+                    key: "/test".to_string(),
+                    value: b"hello".to_vec(),
+                })),
+            },
+        ];
+        
+        let signed = chain.create_entry(&node, ops).unwrap();
+        
+        assert_eq!(chain.len(), 1);
+        
+        // Verify it was written
+        let entries = read_entries(&path).unwrap();
+        assert_eq!(entries.len(), 1);
+        assert_eq!(entries[0].entry_bytes, signed.entry_bytes);
+        
+        std::fs::remove_file(&path).ok();
     }
 }
diff --git a/lattice-core/src/signed_entry.rs b/lattice-core/src/signed_entry.rs
index 042415d..652925b 100644
--- a/lattice-core/src/signed_entry.rs
+++ b/lattice-core/src/signed_entry.rs
@@ -76,6 +76,12 @@ impl EntryBuilder {
         });
         self
     }
+    
+    /// Add a raw operation
+    pub fn operation(mut self, op: Operation) -> Self {
+        self.ops.push(op);
+        self
+    }
 
     /// Build the Entry proto message
     pub fn build(self) -> Entry {